This invention relates to a variable displacement axial piston pump and, more particularly to a timing device which provides for gradual pressure rise and pressure decay as well as the prevention of fluid jet flow which only occurs at the outlet port as the cylinder bores in the cylinder barrel come into communication with the inlet and outlet ports.
The axial piston pump has a cylinder barrel rotatably mounted in a pump housing and is rotated by a drive shaft. The cylinder barrel has plurality of cylinder bores formed therein equally spaced about a common radius, each bore housing a piston which reciprocates as the barrel is rotated. One end of the cylindrical barrel rotates against a fixed valve plate mounted within the housing and which has inlet and outlet ports. Each cylinder has a port adjacent to the valve plate and as the cylinder barrel is rotated, each cylinder port cyclically communicates with the inlet and outlet ports in the valve plate. The pistons are connected through piston shoes to bear against the angled swash plate. As the cylinder barrel is turned by the drive shaft, the piston shoes follow the swash plate and cause the pistons to reciprocate. The inlet and outlet ports in the valve plate are arranged so that the pistons pass the low pressure inlet as they are being pulled out and pass the high pressure outlet as they are being forced back in.
It is important that proper timing be used to communicate the valve plate inlet and outlet ports to the cylinder bores. Proper timing is achieved by selecting the optimal length, depth and width of the metering grooves as well as their proper radial location. Improper timing directly contributes to problems such as high noise level, pressure pulsations, erosion, high yoke moments, poor volumetric efficiency, poor fill capability, and jet flow. Ideally, the fluid in the piston chamber should be decompressed and precompressed to the system pressure level before communicating the piston chamber to the valve plate inlet and outlet ports, however, this is not possible for all conditions of speed and pressure.
In the past, metering grooves have been used to help achieve decompression and precompression. However, jet flow problems occur when, for example, during precompression, the pressure differential between the outlet port and the cylinder bore is great, the pressure within the outlet port being substantially higher than that within the cylinder bore. As the metering groove comes into communication with the outlet port, high pressure fluid is forced back through the small metering groove from the high pressure outlet port and back into the piston chamber creating a fluid jet flow effect. The high velocity of the fluid through the metering groove pulls air out of solution. This aerated fluid, when subjected to the impact of high pressure, will result in high pressure pulsations which are directly related to the noise level of the unit, as well as contributing to erosion of the valve plate as the high velocity fluid flows through the small metering grooves.
To prevent high velocity fluid flow through the small metering grooves which contribute to the above mentioned problems, it is desirable to provide a double notched metering groove at the outlet port, and a long "V" notch metering groove at the inlet port.